54 patients with idiopathic Parkinson's disease and freezing of gait resistant to subthalamic nucleus stimulation and dopaminergic medication will be included into this multicentre randomised controlled double-blinded parallel group clinical trial. The treatment consists of two different stimulation settings using (i) conventional stimulation of the subthalamic nucleus [standard STN] as active comparator and (ii) combined stimulation of active electrode contacts located in both the subthalamic nucleus and substantia nigra pars reticulata [STN+SNr].